1. Technical Field
The present invention relates to methods of producing fine crystals by reactions of metal salts, metal oxides and metal oxyhydroxides under near-critical, critical or supercritical solvent conditions, avoiding thereby many of the difficulties associated with conventional solid state or wet chemistry synthesis, and further relates to the materials so produced. More particularly, the present invention relates to the production of iron-containing mineral crystals of controlled morphology and without noxious byproducts.
2. Description of Related Art
Iron-containing mineral crystals frequently have useful and commercially valuable magnetic properties. In particular, ferrites, which are iron-containing complex oxides, have many technological applications. For example, permanent magnets, ultrahigh frequency electronic components, and magnetic recording media may be manufactured from ferrites, powders of ferrites, and fine crystals of ferrites.
Ferrites may be produced by solid state or wet chemistry synthetic routes. Solid state synthesis typically involves the mixing of dry raw materials for several days, followed by several hours of heating at temperatures in excess of 1000xc2x0 C. The product is then ground to a powder of the required particle size. The grinding process may also take several days. Typically, it is not feasible to add dopants to the final mineral products when the mineral is produced by solid state synthesis. Solid state synthetic methods are discussed in detail in (for example) The Inorganic Chemistry of Materials, Plenum Press, New York, N.Y., 1998 and references cited therein.
Direct synthesis of ferrites in a solvent in the critical domain of temperature and pressure is an attractive alternative to solid state synthesis. The solvent xe2x80x9ccritical domainxe2x80x9d corresponds to temperatures and pressures near the solvent critical temperature and critical pressure, which together constitute the neighborhood of the solvent critical point. The justification for developing methods of producing ferrites and other minerals under near-critical, critical and supercritical solvent conditions is based upon the unique properties of water and other solvents in the critical domain and stems from industry""s need to acquire ready-made minerals efficiently and in a cost-effective manner rather than undergoing the tedious and expensive solid state production route.
In contrast to solid state synthesis methods, direct synthesis of ferrites and other minerals under near-critical, critical and supercritical solvent conditions typically involves mixing of raw material in a solvent for several minutes, followed by several hours of heating at a temperature typically less than 500xc2x0 C. Dopants may be added to the product in situ. The product is typically obtained as a powder of fine crystals with a controllable particle size distribution governed in large part by the residence time in the reactor. Narrow size distributions may readily be obtained by these methods. Grinding is not typically required although may be performed if the final product justifies the additional processing step(s).
Products manufactured according to the present invention have several advantages both in processing and in the characteristics of the final products over conventionally-synthesized mineral powders. A manufacturing process utilizing a powdered mineral as the starting material (reactant) typically involves addition of binders to the powder. A xe2x80x9cgreen bodyxe2x80x9d is typically formed by pressing the powder into the desired shape in a die. The green body is then heated to a high temperature to calcine or sinter the powder, causing the green body further to agglomerate. The sintered body might then be machined to produce the desired final shape. Powder produced according to the direct synthesis procedures of the present invention typically has greater homogeneity than powder obtained by other means. This greater uniformity allows for a much lower sintering temperature than is typically required for a product having less uniformity as typically produced by the methods of solid state synthesis. In addition to saving energy (and cost) in the manufacturing process, lowered sintering temperatures permit the use of reactant materials and/or structures that may not survive the higher sintering temperatures required in connection with the use of conventionally obtained mineral reactants.
The direct synthesis of minerals at near-critical, critical and supercritical solvent conditions utilized in the practice of the present invention is compared with the solid state route in Table 1. A side-by-side comparison of the various processing steps clearly illustrates the advantages of near-critical, critical and supercritical synthesis in terms of faster processing times, reduced energy consumption and a greater degree of processing flexibility in the addition of dopants.
The present invention also has advantages in comparison with wet chemical synthetic methods. Typically, wet chemical methods will be a neutralization method of the general type
2Fe3++M2++ROHxe2x86x92MOxe2x88x92Fe2O3
or an oxidation method of the general type
Fe2++M2++ROH+O2xe2x86x92M1xe2x88x92xFe2xe2x88x92xO4
In both cases, M2+denotes Mg2+, Mn2+, Fe2+, Co2+, Zn2+, Ni2+and similar metal ions, R is typically an organic group and x is a number between 0 and 1. The spinel ferrite particles obtained by the wet chemical neutralization method are typically ultrafine, that is less than about 0.05 micrometer. Such fine particles are difficult to use commercially, such as for commercial sintered ferrites. Some references claim that the wet chemical oxidation method yields particle sizes from 0.05 micrometer to about 1.0 micrometer but practical substantiation of these assertions has not been publicized. See xe2x80x9cFerrites: Proceedings of the International Conference, July 1970, Japan,xe2x80x9d published by University Park Press, Baltimore, London, Tokyo.
Direct synthesis of ferrites in solvents under hydrothermal conditions is discussed in U.S. Pat. Nos. 4,551,260; 4,512,906; 4,671,885; 4,789,494; 4,960,582; 5,863,512. None of these references teaches the use of supercritical solvent conditions. Moreover, the importance of pressure is not widely recognized or taught in these references.
Hayakawa et. al. (U.S. Pat. No. 4,512,906) for xe2x80x9cWet process for preparing ferrites of magnetoplumbite structure in fine particle formxe2x80x9d relates to the preparation of fine particles of ferrites of magnetoplumbite structure by subjecting Fe2O3, Fe3O4, or (FeO)xFe2O3 and at least one compound of a divalent metal selected from Ba, Sr, and Pb to hydrothermal treatment in an aqueous alkali solution at T=80xc2x0 C. to 360xc2x0 C. Hexagonal, plate-like crystals result. The metal salts and metal oxides used as reactants in the present invention, as described in detail below, differ from the reactants of this patent.
Hayakawa et. al. (U.S. Pat. No. 4,551,260) xe2x80x9cWet process of preparing fine particles of ferrite of magnetoplumbite structurexe2x80x9d relates to the preparation of magnetoplumbite ferrites substituted with Fe, Cu, Zn, Co, Ni, Mn and Mg. All the ferrites contain Sn, using different reagents and producing different products from the present invention.
Gaud et. al. (U.S. Pat. No. 4,671,885) for xe2x80x9cProcess for the preparation of magnetized hexagonal ferrites, use of these ferrites as base materials for magnets or for magnetic recordingxe2x80x9d relates to the preparation of magnetic hexagonal ferrites with the iron optionally substituted hydrothermally at P less than 500 atm, T greater than 100xc2x0 C. by reacting one alkali metal ferrite and a compound which releases an alkaline-earth metal ion in water and optionally one or more alkali metal salts of metal oxides. The present invention does not require alkali metal ferrites as reagents.
Aoki et. al. (U.S. Pat. No. 4,789,494) for xe2x80x9cHydrothermal process for producing magnetoplumbitic ferritexe2x80x9d relates to a hydrothermal process for producing magnetoplumbitic ferrite powder in water at 100xc2x0 C. less than T less than 400xc2x0 C. and in the presence of an alkali.
Ado et. al. (U.S. Pat. No. 4,960,582) for xe2x80x9cProcess for producing microcrystalline Co/Ti-substituted barium ferrite plateletsxe2x80x9d relates to a hydrothermal process to provide precursors of spinel structure nickel ferrite-modified, Co/Ti-substituted barium ferrites, followed by calcining to produce the ferrites. Calcining is not required to produce the reaction products of the present invention.
Iwasaki et. al. (U.S. Pat. No. 5,863,512) for xe2x80x9cProcess for preparing layered rock salt type lithium ferrite by mixed alkali hydrothermal methodxe2x80x9d relates to a hydrothermal process for preparing LiFeO2 with a rock salt type structure from iron salts, iron hydroxides, iron oxide hydroxides (FeOOH), or iron in an aqueous solution containing LiOH and NaOH or KOH.
The present invention relates to methods of producing fine crystals by reactions of metal salts and metal oxyhydroxides under near-critical, critical or supercritical solvent conditions, avoiding thereby many of the difficulties associated with conventional solid state or wet chemistry synthesis, and further relates to the materials so produced. The metal in the reactants is typically in either a divalent or a trivalent state. The synthetic crystals (typically mineral crystals) may be formed by the methods of the present invention in numerous crystal structures including but not limited to: ferrite and non-ferrite spinel, hexagonal and garnet-like crystal structures. The crystals produced by the present invention also have morphologies, based upon the fourteen (14) lattice types in three (3) dimensions (for example, see Handbook of Chemistry and Physics, 80th Edition, 1999-2000, p. 12-9), such morphologies have favorable properties for compaction into green bodies for subsequent sintering into near-net-shapes, approaching maximum theoretical densities.
Reactants and reaction conditions are described that yield various crystal products (typically mineral) including but not limited to trevorite and zinc substituted trevorite having the general stoichiometry Nix,ZnyFe2O4 in which x and y are fractions between zero (0) and one (1) that sum to approximately one (1). Examples include x=0.95, y=0.05 and x=0.6, y0.4. A reactant especially useful in the practice of the present invention is a mineral form of FeOOH commonly referred to as goethite. The resulting (typically mineral) products have surface area to mass ratios that can be adjusted by varying the reaction conditions. Preferable surface area to mass ratios of around 9 to 10 square meters per gram are obtained. The methods of the present invention typically produce fine crystals which can be distinguished from naturally occurring crystals and from crystals produced via solid state synthetic routes or by wet chemistry synthesis (typically neutralization or oxidation methods). Salient features distinguishing the crystals of the present invention from crystals produced by solid state or wet chemical routes include, but are not limited to, morphology, size distribution, surface-to-mass ratio, magnetic properties, range of applications, and low cost.
The present invention permits dopants to be added to the reactants for incorporation into the (typically mineral) products. Adjustable surface area to mass ratios of the final products without grinding are another advantage of the present invention. Energy savings and lower costs are the typical result when crystals are produced according to the present invention, in comparison with other ways to obtain the crystals (in particular, the solid state synthetic route).